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Inside the FMS: Requirements for RNP 1

Inside the FMS

The Procedure States RNP 1... Can I Fly it?

The answer is, “it depends”.

The best source to determine if a specific Honeywell FMS meets the ICAO PBN requirements for RNP 1 is Service Information Letter (SIL) D201707000018. This SIL can also be found on the Honeywell Pilot Gateway.

However, it is important to understand why some FMSs can and cannot fly RNP 1 procedures. We will first begin by inspecting the requirement differences for RNAV and RNP procedures. The only difference between RNAV and RNP is the requirement for on-board performance monitoring and alerting (OBPMA). Now, what exactly constitutes OBPMA? According to the ICAO PBN Manual (doc 9613):

“On-board Performance monitoring: The aircraft, or aircraft and pilot in combination, is required to monitor the TSE, and to provide an alert if the accuracy requirement is not met or if the probability that the TSE exceeds two-times the accuracy value is larger than 10-5.”

There are three types of position errors that roll-up to total system error (TSE). They are:

  • 1) NSE – Navigation System Error: the difference between the aircraft’s estimated position and actual position. It is measured via the RNP vs. EPU display on the MCDU (Figure 2)
  • 2) FTE – Flight Technical Error: relates to the crew or autopilot’s ability to follow the defined path or track, including any display error. It is measured by a CDI that is appropriately scaled for the RNP value required for the phase of flight. For example, RNP 1 requires a scaled CDI display where 2 dots of deviation = 1 nm of crosstrack error.
  • 3) PDE – Path Definition Error: occurs when the path defined in the RNAV system does not correspond to the desired path. Use of an RNAV system for navigation presupposes that a defined path representing the intended track is loaded into the navigation database. For this reason, PDE is assumed to be negligible.

So, if an FMS has the ability to monitor all three of those errors, it meets the requirements for OBPMA and can fly RNP procedures.

But wait! There’s more. This is where the answer of “it depends” really comes from in response to this article’s title. We will examine a particular FAA Advisory Circular that has caused some confusion among operators when it comes to flying RNP 1 procedures.

Paragraph 2.1 in AC 90-105A states:

“The guidance contained in this advisory circular (AC) applies to all operators conducting Required Navigation Performance (RNP) operations under Title 14 of the Code of Federal Regulations (14 CFR) parts 91, 91 subpart K (91K), 121, 125, 129, and 135 within the U.S. National Airspace System (NAS). It also applies to all U.S. operators conducting RNP operations offshore as defined by the Federal Aviation Administration (FAA) Order JO 7400.9( ), Airspace Designations and Reporting Points; and oceanic and remote continental airspace. This AC also applies to all U.S. operators conducting RNP operations in foreign airspace, where the foreign authority has implemented RNP in accordance with International Civil Aviation Organization (ICAO) Doc 9613.”

Paragraph C.2.3 provides a “grandfather clause” and addresses qualified avionics equipment and airworthiness approval for RNP 1 (terminal) operations. Several TSOs and other Advisory Circulars to meet qualification are referenced in C.2.3. Discussing them individually is outside the scope of this article. Readers are encouraged to reference SIL D201707000018 to determine their eligibility.

So, according to AC 90-105A, if an aircraft has equipment installed that qualifies under paragraph C.2.3 then…

  • > It may fly RNP 1 procedures in the United States
  • > It may fly RNP 1 procedures offshore, if flown by a US operator
  • > It may fly RNP 1 procedures in foreign airspace, if flown by a US operator

It should be noted that some Part 91 operations outside of the Domestic US NAS require an LOA; you should consult the Foreign State AIP for their operational requirements regarding an LOA to fly RNP 1 procedures in their airspace.

Also, it has been reported that foreign authorities implementing RNP in accordance with ICAO doc 9613 (including, but not limited to, PR of China and Australia) have not allowed US operators to fly RNP 1 procedures utilizing the “grandfather clause” in AC 90-105A. The cited reason for this is that previously approved RNAV 1 equipment under this AC does not have the performance monitoring requirement for RNP per ICAO doc 9613, as discussed earlier.

These RNP 1 procedures can be identified by the notes in the chart. Some Foreign State AIP’s are more specific than others in calling out the requirement for RNP 1 meeting ICAO standards. PR of China, for example, is very specific in calling out the ICAO RNP definition in the notes (Figure 4).

Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation

Australia, on the other hand, simply lists “RNP 1” in the notes section (Figure 5). Crews should note that in Australia, the procedure title does not include the words “RNP” and illustrates that sometimes the notes section is the only way to identify the RNP requirement for these procedures.

Copyright Jeppesen, Inc. – Used by Permission – Not for Navigation